Page 42 - DCAP406_DCAP_207_Computer Networks

Unit 3: Physical Layer

Distance: Twisted pair is distance limited. As distance between network elements increases, Notes
attenuation (signal loss) increases and quality decreases at a given frequency. As bandwidth
increases, the carrier frequency increases, attenuation becomes more of a issue, and
amplifiers/repeaters must be spaced more closely.

Security: Twisted pair is inherently an insecure transmission medium. It is relatively
simple to place physical taps on UTP. Additionally, the radiated energy is easily intercepted
through the use of antennae or inductive coils, without the requirement for placement of
a physical tap.
Cost: The acquisition, deployment and rearrangement costs of UTP are very low, at least
in inside wire applications. In, high-capacity, long distance applications, such as inter-
office trunking, however, the relative cost is very high, due to the requirements for
trenching or boring, conduit placement, and splicing of large, multi pair cables.
Additionally, there are finite limits to the capacity and other performance characteristics
of UTP, regardless of the inventiveness of technologists. Hence, the popularity of
alternatives such as microwave and fibre-optic cable.

Applications: UTP’s low cost including recently developed methods of improving its
performance has increased its application in short-haul distribution systems or inside
wire applications. Current and continuing applications include the local loop, inside wire
and cable, and terminal-to-LAN. Generally speaking, UTP no longer is deployed in long
haul or outside the premises transmission systems.

The additional cost of shielded copper limits its application to inside wire applications.
Specifically, it generally is limited to application in high-noise environments. It also is deployed
where high frequency signals are transmitted and there is concern about either distance
performance or interference with adjacent pairs. Examples include LANs and image transmission.

3.2.2 Coaxial Cable

The main limiting factor of a twisted pair cable is caused by a phenomenon known as the skin
effect. As the frequency of the transmitted signal increases, the current flowing in the wires
tends to flow only on the outer surface of the wire, thus using the less of the available cross
section. This increases the electrical resistance of the wires for higher frequency signals leading
to higher attenuation. In addition, at higher frequencies, more signal power is lost as a result of
radiation effects. Hence for applications that demand higher frequencies, another type of
transmission medium must be used. Coaxial cable minimizes both these effects.
Coaxial Cable as shown in Figure 3.4 is a very robust shielded copper wire two-conductor cable
in which a solid centre conductor runs concentrically (coaxial) inside a solid outer circular
conductor. This forms an electromagnetic shield around the former that serves to greatly improve
signal strength and integrity. The two conductors are separated by insulation. A layer of dielectric
(nonconductive) material, such as PVC or Teflon then protects the entire cable.

It comes under the category of a bounded media and is still an effective medium to use in data
communication. Coaxial cable includes shield for improved performance and therefore is
expensive. Cable TV networks use coaxial cable. Local Area Networks can operate over coaxial
cable to the 10BASE5, 10BASE2 and 10BASET specifications. In general, coaxial cable enables
longer distance transmission at higher data rates than twisted pair cable but is more expensive.
There are two types of coaxial cables:

Baseband: It transmits a single signal at a time at very high speed. The signal on baseband
cable must be amplified at a specified distances. It is used for local area networks.
Broadband: It can transmit many simultaneous signals using different frequencies.

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